Alzheimer's disease (AD) is the most common cause of age-related cognitive decline, affecting greater than 12 million individuals worldwide (Citron M (2002) Nat. Neurosci 5, Suppl 1055-1057). The earliest stages of the disease are characterized by a progressive loss of memory with associated cognitive decline and language and behavioural deficits. In the later stages of the disease, patients develop global amnesia and have greatly reduced motor function. Death typically occurs 9 years following diagnosis and is often associated with other conditions, typically pneumonia (Davis K. L. and Samules S. C. (1998) in Pharmacological Management of Neurological and Psychiatric Disorders eds Enna S. J. and Coyle J. T. (McGraw-Hill, New York pp 267-316)). Current therapies represent symptomatic approaches, focussing on alleviating the cognitive impairment and ameliorating the behavioural symptoms associated with the progressing disease etiology. In practice these treatments provide only a short lived cognitive benefit with the level of cognitive impairment reported only to last up to 2 years. The potential for a disease-modifying therapy that slows and possibly halts the progression of the disease is enormous. Such approaches would provide radical and sustained improvements to the quality of life of patients and importantly their careers as well as reducing the huge overall healthcare costs of this disease.
Clinical diagnosis of Alzheimer's disease is currently based upon a combination of physical and mental tests which lead to a diagnosis of possible or probable Alzheimer's disease although diagnostic biomarkers and imaging are under investigation (Sonnen et al. (2007) Expert Rev Neurotherapeutics 7(8): 1021-1028; Lockhart et al (2007) Brain 130: 2607-2615). At post mortem the disease is confirmed by well characterised neurological hallmarks in the brain, which include the deposition of Aβ in parenchymal plaques and cerebral vessels, intraneuronal formation of neurofibrillary tangles, synaptic loss and loss of neuronal subpopulations in specific brain regions (Terry, R D (1991) J Neural Trans Suppl 53: 141-145).
A plethora of genetic, histological and functional evidence suggests that the β-amyloid peptide (Aβ) is key to the progression of Alzheimer's disease (Selkoe, D. J. (2001) Physiological Reviews 81: 741-766) although it has become less clear recently whether the actual Aβ deposits observed on post mortem examination are the true cause of the cognitive decline (Ferreira S T (2007) Life 59(4-5): 332-345). Aβ is known to be produced through the cleavage of the β-amyloid precursor protein (also known as APP) by an aspartyl protease enzyme known as BACE1 (also known as β-secretase, Asp2 or Memapsin-2) (De Strooper, B. and Konig, G. (1999) Nature 402: 471-472). In addition to the parenchymal and vascular deposition, soluble oligomeric forms of Aβ have been postulated to contribute to the onset of AD and they may affect neuronal function initially by impairing synaptic function (Lambert et. al. (1998) Proceedings of the National Academy of Science, U.S.A. 95: 6448-6453; Kayed et al (2003) Science 300:486-489; Cheng et al (2007) J Biol Chem 282(33):23818-23828; Ferreira et al (2007) Life 59:332-345). Although insoluble amyloid plaques are found early in AD and in mild cognitive impairment (MCI), the levels of soluble Aβ aggregates (sometimes referred to as oligomers or Aβ-derived diffusible ligands (ADDLs)) are also increased in these individuals, and soluble Aβ levels correlate better with neurofibrillary degeneration, and the loss of synaptic markers than do amyloid plaques (Naslund et. al. (2000) J Am Med Assoc 283: 1571-1577, Younkin, S. (2001) Nat. Med. 1: 8-19). In addition these oligomers may represent precursors on the route to fibril formation and their removal or neutralisation may prevent toxic effects and fibril formation (Ferreira ST (2007) Life 59(4-5): 332-345; Gong Y (2003) PNAS 100:10417-10422). Despite these findings the highly amyloidogenic Aβ42 and amino-terminally truncated forms Aβx-42 are the predominant species of Aβ found in both diffuse and senile plaques (Iwatsubo, T (1994) Neuron. 13:45-53, Gravina, S A (1995) J. Biol. Chem. 270:7013-7016) and the relative levels of Aβ42 appear not only to be a biomarker for AD but also a key regulator of Aβ aggregation into amyloid plaques. Aβ42 has been shown to aggregate more readily than other Aβ forms in vitro (Jarrett, J T (1993) Biochemistry. 32: 4693-4697) and as such Aβ42 has been implicated as the initiating molecule in the pathogenesis of AD (Younkin S G, (1998) J. Physiol. (Paris). 92:289-292). Although Aβ42 is usually a minor product of APP metabolism, small shifts in its production are associated with large effects on Aβ deposition and therefore it has been postulated that reduction of Aβ42 alone may be an effective way of treating AD (Younkin S G, (1998) J. Physiol. (Paris). 92:289-292; Levites et al (2007) J Clin Invest. 116(1):193-201). In support of this, mutations in the amyloid precursor protein (APP) and presenilin genes have been reported to predominantly increase the relative levels of Aβ42 and therefore shortening the time to onset of Alzheimer's disease (AD) (Selkoe D. J., Podlisny M. B. (2002) Annu. Rev. Genomics Hum. Genet. 3:67-99). It should be stressed however, that the rate of amyloid deposition is also dependent on overall amyloid levels, catabolism and the efficiency of Aβ clearance from the CNS which has been shown to be negatively influenced by age and elevated amyloid levels as found in AD (Deane et al (2005) J Neurosci 25(50):11495-11503; Wang et al (2006) Drug Discovery Today 11(19/20): 931-938). In this respect it has become increasingly apparent that the transport of Aβ between the central nervous system (CNS) and plasma plays a major role in the regulation of brain amyloid levels (Shibata, et al (2000) J Clin Invest 106: 1489-1499), Aβ being rapidly transported from the CNS to plasma by transport mechanisms such as LRP-1 and Aβ being rapidly imported from the plasma to the CNS by binding to RAGE (Zlokovic B V (2004) J Neurochem 89: 807-811). Therefore active vaccination with Aβ peptides or passive administration of specific Aβ antibodies that bind peripheral Aβ and therefore alter the dynamic equilibrium between the plasma, CSF and the CNS are being developed. Indeed there are now numerous studies that have demonstrated that both these approaches can lower Aβ levels, reduce Aβ pathology and in some cases provide cognitive benefit in various transgenic models of amyloidosis. Limited studies have also been conducted in higher species (Lemere, C A (2004) Am J Pathology 165: 283-297; Gandy, S (2004) Alzheimer Dis Assoc Disord 18: 44:46).
Animal models of amyloid deposition have been generated by overexpressing mutant human transgenes in mice. Mice overexpressing single human APP transgenes typically develop cerebral plaque-like β-amyloid deposits from 12 months of age (Games D. et al., (1995) Nature 373: 523-527; Hsiao K. et al., (1996) Science 274: 99-102)), while mice carrying both mutant human APP and presenilin-1 (PS-1) transgenes typically develop cerebral plaque-like β-amyloid deposits as early as 2 months of age (Kurt M. A. et al., (2001) Exp. Neurol. 171: 59-71; McGowan E. et al., (1999) Neurolbiol. Dis. 6: 231-244). These considerable biological differences in the various transgenic mouse models used have made it difficult to compare the pharmacology and the efficiency of different approaches. There is no real consensus in the field how immunotherapies targeted at β-amyloid and its various forms actually work. It is highly likely that different antibodies with different binding properties have variable outcomes in those animal models. It seems also reasonable that antibodies may act by multiple mechanisms and that the different modes of action that have been described are not mutually exclusive (Levites et al (2007) J Clin Invest. 116(1):193-201).
The first immune therapy targeting brain amyloid in the clinic was Elan/Wyeth's AN-1792, an active vaccine. This treatment was terminated following the development of clinical signs consistent with meningoencephalitis. Subgroup analyses suggested that treatment slowed the decline of cognitive function (Nature Clin Pract Neurol (2005) 1:84-85). Post-mortem analysis of patients also showed evidence of plaque-clearance (Gilman S. et al, (2005) Neurology 64 (9) 1553-1562).
Bapineuzumab (AAB-001, Elan/Wyeth), a passive monoclonal antibody, is in development.
Other diseases or disorders characterised by elevated β-amyloid levels or β-amyloid deposits include mild cognitive impairment (Kelley B J (2007) Neurologic Clinics 25 (3), 577-609), hereditary cerebral hemorrhage with β-amyloidosis of the Dutch type, cerebral β-amyloid angiopathy and various types of degenerative dementias, such as those associated with Parkinson's disease, progressive supranuclear palsy, cortical basal degeneration and diffuse Lewis body type of Alzheimer's disease (Mollenhauer B (2007) J Neural Transm e-published 23 Feb. 2007, van Oijen, M Lancet Neurol. 2006 5:655-60), Down's syndrome (Mehta, P D (2007) J Neurol Sci. 254:22-7), Age-related macular degeneration (AMD) (Johnson L V et al (2002) PNAS USA 99: 11830-11835; Anderson D H et al (2004) Exp Eye Res 78: 243-256), “Glaucoma type” diseases (Guo L et al (2007) Proc Natl Acad Sci USA 104:13444-13449) and Aβ dependent cataract formation (Goldstein L E et al (2003) Lancet 361: 1258-1265; Li G et al (2003) Mol Vision 9: 179-183).
Age-related macular degeneration (AMD) is the leading cause of blindness in the developed world. There are two major clinical presentations of AMD. Atrophic (dry) AMD is characterised by the degeneration of retinal pigment epithelial (RPE) and neuroretina. The early stages of atrophic AMD are associated with the formation of drusen, under the RPE cell layer. Early atrophic AMD can progress to an end stage disease where the RPE degenerates completely and forms sharply demarcated areas of RPE atrophy in the region of the macula: “geographic atrophy”. In this form of the disease, the degeneration of RPE results in the secondary death of macular rods and cones and in these cases this leads to the severe age-related vision loss. A proportion of AMD patients develop what can either be regarded as a different form or a further complication of the disease. Approximately 10-20% of AMD patients develop choroidal neovascularisation, (CNV). When this occurs the form of the disease is known as “wet AMD” and this can be associated with some of the most severe vision loss. In wet AMD, new choroidal vessels grow through breaks in Bruch's membrane and proliferate into and under the RPE and neuroretina. In typical cases, atrophic AMD develops in the eye before the development of the wet form, however, on infrequent occasions, the neovascular form can develop in the absence of prior development of the atrophic form. In both forms of the disease, vision loss occurs due to the death of photoreceptor cells, although in wet AMD internal bleeding from the leaky vessels formed during CNV also causes vision loss. In terms of therapy for AMD there has been some progress in developing novel treatments to address some aspects of wet AMD, in particular the reduction of leaky vessel bleeding from CNV by various molecules that inhibit either VEGF (vascular endothelial growth factor) or the VEGF receptor signalling pathway. However, currently there are no definitive means of treatment for the very prevalent atrophic form of AMD nor to prevent the progression of early dry AMD either to geographic atrophy or to wet AMD, (Petrukhin K (2007) Expert Opin Ther Targets 11: 625-639) Although the exact mechanisms that cause the production of Aβ in RPE and the exact mechanism or mechanisms by which Aβ acts to influence AMD are not completely understood, the evidence implies that clearing of Aβ by agents that bind and potentially neutralise or just remove Aβ may provide a possible route to clearing drusen in AMD, reducing complement activation in AMD, reducing RPE atrophy and potentially reducing the induction of VEGF expression in RPE and its localisation at high levels around drusen. Such therapy could therefore provide means of preventing, delaying, attenuating or reversing the loss of vision due to AMD and its progression to geographic atrophy and/or exudative AMD. This may result in decreased levels of Aβ containing drusen and/or local Aβ in the surrounding environment of the RPE and thereby interfere in both the early and later stages of AMD and treat the underlying cellular decline that causes the loss of vision.
Some recently published data has shed light on the interaction of complement proteins and amyloid beta in the generation of AMD (Wang, J. et al., (2008) J. Immunol. 181: 16651-6). Amyloid beta has been shown to bind to complement factor I, the co-factor that with factor H is responsible for the breakdown of complement protein C3 from the C3b form to its inactive form, iC3b, (Wang, J. et al., 2008). The results from the recently published in vitro study were suggested to support a hypothesis where amyloid beta activates the complement system within drusen by blocking the function of complement factor I leading to a low-grade, chronic inflammation in sub-retinal tissues; thus linking four of the factors associated with the development of AMD: inflammation, complement activation, amyloid beta deposition and drusen, (Wang, J. et al., 2008). Such direct evidence for the effect of amyloid beta in activating the alternative complement pathway by potentially competing with complement factor H for binding to complement factor I has not previously been documented, (Wang, J. et al., 2008).
“Glaucoma type diseases” is a term used for a group of diseases that can lead to damage to the eye's optic nerve and result in blindness. It is a major cause of blindness in the world caused ultimately by increased intraocular pressure (IOP) and decreased visual acuity. The link between IOP and how this leads to apoptosis of the retinal ganglion cells (RGC) is not well understood. High IOP alone can induce apoptosis (Cordeiro M F et al (2004) Proc Natl Acad Sci USA 101:13352-13356; Quigley H A et al (1995) Invest Ophtalmol Visual Sci 36:774-786) but in itself is not the only cause of cell death of the optic neurons. In addition it has been observed that the vision can continue to deteriorate even after the normalisation of the IOP following treatment with eye pressure lowering agents (Oliver J E et al (2002) Am J Ophthamol 133:764-772).
Recently there have been reports linking the potentially cytotoxic effects of β-amyloid to apoptosis of RGCs in glaucoma (McKinnon S J et al (2002) Invest Ophtamol Visual Sci 43:1077-1087). In animal models of glaucoma it has been demonstrated that caspase-3 protease is activated in RGCs which leads to abnormal processing of amyloid precursor protein (APP) by caspase-3 generating potentially toxic fragments of APP including β-amyloid (McKinnon et al (2002); Cheung Z H et al (2004) Mol Cell Neurosci 25:383-393). Amongst other cells, RGCs have been shown to express APP and this therefore appears a plausible source of β-amyloid. Both elevated levels of APP and elevated levels of β-amyloid have been implicated with activating caspase-3 although this has been observed primarily in in vitro systems. It is unclear whether APP levels in the RGCs are also increased in glaucoma thus contributing to the generation of even more β-amyloid in a positive feed back mechanism. Even more recently, the involvement of β-amyloid with apoptosis of RGCs in a rat model of glaucoma has been suggested (Guo et al (2007)). Several agents targeting β-amyloid or β-amyloid production were tested and showed a reduction of retinal ganglion cell death in vivo with a possible mild enhancement effect when all three treatments were used together. The largest effect was seen by using an anti-β-amyloid antibody which almost matched the effects seen with all three agents together.
Although the exact mechanisms that cause the production of β-amyloid in RGCs and the connection with IOP are not completely understood, the evidence implies that clearing of β-amyloid by agents that bind and potentially neutralise or just remove β-amyloid may provide a possible route to preventing RGC apoptosis in glaucoma and therefore provide means of delaying, attenuating or reversing the loss of vision in glaucoma. This may result in decreased levels of β-amyloid in the RGCs and surrounding environment and thereby address the underlying cellular decline that causes the loss of vision.
β-Amyloid may play a role in other ocular diseases and has been associated with the formation of supra-nuclear cataracts especially in those seen in AD patients and the components of the Aβ generation and processing pathway are present in the lens (Goldstein L E, et al., (2003); Li G, et al., (2003)). The therapeutic approaches described for intervention in AMD and glaucoma-type diseases may therefore be applicable to the prevention of Aβ dependent cataract formation.
WO 2008/110885 relates to methods of treating ophthalmic diseases with inhibitors directed against amyloid-β peptide. Specifically, antibody 6G which binds to an epitope on Aβ1-40 that seems to include 25-34 and 40 is disclosed.